I built mine by putting a couple of the spacers back on the freewheel and threading the sprocket back on. Then I got the welder and threw a couple spot welds on to lock up the freewheel and a couple on the sprocket to lock it down as well. Almost as good as duct tape and a coat hanger. Hard to see in the pic.
I put this on the Grunge Rudge. Loctite, heavy on the wrench, and a bottom bracket nut. I also redished the wheel and cut off the extra axle length. For the cost of $10 for the track gear I found that fixie's aren't for me. :roll:
Next think you need to think about is chaintension on a single speed. Fairly easy with horizontal dropouts. You will need a chain tensioner for vertical dropouts.